Field of the Invention
The invention relates to a circuit configuration for driving an electrical drive unit in a motor vehicle having a DC voltage source and having a control circuit in the form of a current bridge and a bridge driver for voltage commutation, whereby the control circuit is located between the DC voltage source and the electrical drive unit.
Such circuit configurations with control circuits used for voltage commutation are known in the field of motor vehicle electronics. They comprise a current bridge constructed using MOSFETs, for example. The MOSFETs are suitably addressed by a bridge driver, so that the direction of rotation of a DC motor connected to the current bridge can be reversed. Owing to heating of the power switches, caused by large motor currents, on the one hand as a result of ohmic on-state losses and on the other hand as a result of switching losses during pulsed operation, it must be ensured that the power switches do not overheat on account of continuous operation, as this could lead to destruction. For this reason, it is necessary to monitor the temperature of the power switches.
Various possibilities have become known in the art with regard to the design of temperature monitoring circuits.
In monolithic integrated circuits, there is close thermal coupling between the bridge driver and the bridge circuit. It is therefore sufficient to mount a temperature sensor on the bridge driver. The temperature of the power switches is then returned as an approximation by the temperature sensor. Monolithic integrated circuits can be produced only up to a maximum of 10 A, however. This current is not sufficient for many applications.
If the bridge circuit is realized discretely, a temperature sensor can be mounted on each power switch. Methods exist for temperature monitoring e.g. using so-called chip-on-chip assembly of a temperature sensor on a power transistor, or there are intelligent power transistors having integrated temperature detection. However, one disadvantage is that the evaluation circuit which indicates whether or not a limit temperature has been reached is usually only digital. This may result in its operation possibly being terminated immediately in the event of thermal overloading. This is generally not desirable, however. Furthermore, this solution is complex and expensive.
European patent No. EP 0 578 958 B1 discloses an overload protection circuit for a transistor. That invention is based on recognition of the fact that, in closed-loop operation, the current through the transistor is constant and hence the power loss is proportional to the voltage drop across the transistor. If the voltage drop is supplied to an integrator whose time constant is matched to the thermal time constant of the transistor, the voltage at the integrator is a measure of the energy loss or the temperature of the transistor. The integrator voltage is compared with a reference value and, if this reference value is exceeded, is used as a turn-off criterion for the transistor, for example. Temperature monitoring using this method requires every transistor to be monitored. This is complex and expensive.